SilverStripe\Subsites\Extensions\LeftAndMainSubsites::onBeforeInit
Redirect the user to something accessible if the current section/subsite is forbidden.
This is done via onBeforeInit as it needs to be done before the LeftAndMain::init has a chance to forbids access via alternateAccessCheck. If we need to change the subsite we force the redirection to /admin/ so the frontend is fully re-synchronised with the internal session. This is better than risking some panels showing data from another subsite.
Signature
public function onBeforeInit()